It is known that certain ligands can combine with metal ions, producing a complex that removes the ion from solution. For example, this type of ligand chemistry has been practiced using crown ethers to capture and remove cobalt ions.
In the past, ligands have been bound to silica gel and thus been immobilized. As a matter of academic achievement, silica gel has been modified to bind 2,2 dipicolylamine (DPA). Studies were done to characterize the modified silica and determine the equilibria with H+and Cu.sup.2+ in solution. D. Sallin, Modification d'une silice par greffage de las dipicolylamine et etude de ses interactions avec le Cuivre(II), Diss. No. 929, Universite' de Fribourg, (1987). The modified silica also is described in V. Dudler, L.F. Lindoy, D. Sallin and C.W. Schlapfer, J. Austral. Chem. 40 (1987) 1557. However, no specific application for this modified surface was investigated or known.
Generally, it has been thought that a ligand should have a high affinity constant for the metal cation to be removed from solution. There has been no reliable way of predicting what ligands would have a high affinity, except by experimentation. A further problem has been that as pH lowers with increasing capture of cations, the competitive reaction capturing the H.sup.+ ion instead of the metal cation shifts in favor of the H.sup.+ ion. Thus, it is increasingly inefficient over time to use ligands for removal of metal cations, as pH drops. Further, in order to be useful in an industrial situation, it has been necessary that the ligand not only capture the ion, but that the ion be removeable from the ligand under readily achievable conditions.
It would be desirable to discover ligands capable of capturinq cations of transition and heavy metals. In particular, it would be desirable to discover ligands capable of efficiently capturing cations even when the ligand is bound to a substrate surface. Further, it would be desirable to discover ligands that maintain a competitive ability to capture cations over a broad range of pH, especially in the low pH range.
To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the ligand and modified silica surface and method of use of this invention may comprise the following.